438 MINERALOGY 



and with the addition of alkali and water form micas. Artificial 

 beryl has been formed by fusing the constituent oxides with boric 

 acid. 



ORTHOSILICATES 



SODALITE GROUP 



COMPOSITION GRAVITY INDEX or RE- 



FRACTION 



Sodalite Na 4 (AlCl)Al 2 (Si0 4 ) 3 2.14-2.30 1.4827 



Haiiynite (Na. Ca) 2 (NaSO 4 Al)Al 2 (SiO4)3 2.4-2.5 1.496 



Noselite Na4(NaSO 4 Al)Al 2 (Si0 4 )3 2.25-2.4 



Lazurite Na 4 (NaS 3 . Al)Al 2 (Si0 4 ) 3 2.38-2.45 



The four species included in this group are all alkali aluminium 

 orthosilicates containing either chlorine or sulphur. They are all 

 isometric in symmetry, have a low specific gravity and a hardness of 

 5.5-6, and a vitreous or greasy luster ; Crystal forms are the cube, 

 rhombic dodecahedron, and octahedron, but generally massive or 

 granular; Cleavage, dodecahedral ; Brittle; Fracture, conchoidal 

 to uneven. Color, white, gray, yellow, blue, green, or brown; 

 Streak, white or pale blue. Index of refraction very low. All iso- 

 tropic, haiiynite may in cases show a weak double refraction. 



B.B. Fuse at 3.5 to 4.5. All gelatinize with HC1 ; the solu- 

 tion freed of silica and slowly evaporated will yield cubic crystals 

 of salt (NaCl). Lazurite when dissolved in HC1 yields hydrogen 

 sulphide and usually glows with a blue flame when first heated 

 in the O. F. 



When haiiynite is dissolved in HC1, the solution freed of silica, 

 and the Al precipitated with ammonia and filtered out, the filtrate 

 will yield a white precipitate with ammonium carbonate (Ca) ; 

 the other three members of the group containing no calcium will 

 not yield a precipitate. Noselite is distinguished from sodalite 

 by yielding a sulphur reaction when fused with soda and a little 

 coal dust, then placed on silver. 



General description. These four minerals are associated in 

 nature and occur as constituents of those lavas low in silica and 

 high in their alkali content, where nepheline and leucite are their 

 companions. Haiiynite and noselite are found in the lavas of recent 

 geological date and are very rare in the coarsely crystalline elseolite 

 syenites, here sodalite alone of the group occurs. Lazurite occurs 

 in crystalline limestone and is a product of contact metamorphism. 



